Anywho tis most defiantly a browser/java problem, I have the game which works fine in linux, but as you say I get a grey screen when trying the demo. I'm using Firefox, probably icetea or whatever the java firefox plugin is.

But I can confirm Minecraft defiantly works on linux. If memory serves you have to install the 'official' orcale java (1.7 I think, whatever the latest one is) cos the 'free' javas don't have the stuff minecraft needs (hence grey screen) to run

Ok, I've installed the proper version of Java and get the Java security check up. I then get a screen saying "Minecraft updating...Done loading" but it sticks at that point. Well at least it I have an official Java installed now. Like Bob said, shouldn't give me a hard time now when I buy the game.